Since efficient encryption algorithms, such as the Advanced Encryption Standard (AES), can not be proven to be secure it is always desirable to bolster security further. One method of encryption called AlphaEta was disclosed in U.S. patent applications Ser. Nos. 10/674,241, 10/982,196, and 11/404,329 by the same team of inventors as the present invention; all of those applications are fully incorporated herein by reference. The method performs physical-layer encryption using a combination of traditional algorithms and random noise. Being a method of physical encryption, the transmitted signal modulation is controlled by the AlphaEta protocol. This is unlike typical encryption methods which usually manipulate the parity of binary data according to an encryption algorithm (see, for example, Practical Cryptography by N. Ferguson and B. Schneier, Wiley Publishing, 2003).
The basic implementation of AlphaEta is described in U.S. patent application Ser. No. 10/674,241; and a method to synchronize the encryption/decryption signals is described in U.S. patent application Ser. No. 11/404,329. In AlphaEta, a short secret key is shared between the transmitter (Alice) and the receiver (Bob). This key seeds an extended key generator (EKG), which extends it into a very long sequence of bits, called the extended key. For every data symbol to be transmitted, where the number of data bits in each symbol is N, several extended key bits are grouped and used as a running key to extend the N-bit symbol to a larger M-bit symbol, where M>N. The M-bit symbol is implicitly or explicitly corrupted by a small amount of noise such that the 2M possible symbols can not be uniquely determined. The signal is transmitted to a receiver, which uses a matched EKG to translate the M-bit symbol back into an N-bit symbol which then allows for the reception of the N-bit data with a low bit error ratio.
Typically AlphaEta is implemented directly on an optical signal so as to take advantage of quantum noise as the random noise source (random noise is exploited for security purposes in AlphaEta). Such a method is useful for optical point-to-point links or in all-optically switched networks. The use of AlphaEta in Wavelength Division Multiplexed (WDM) optical systems is described in U.S. patent application Ser. No. 10/982,196 by the same team of inventors. Although such systems are important, they represent only a fraction of communication systems currently employed. In particular, electronically switched networks which can function over optical, wireline, and RF wireless media are of significance. It is desirable for nodes in secure packetized networks to be able to inspect some packet information in order to determine if and how to decrypt the signal, such as disclosed by Kirby, et al. in U.S. Pat. No. 5,898,784. If the packet is to be re-forwarded, there is a need that the node determines to which (switched) port to send the packet and, for security reasons, to do so without fully decrypting the data. Such advanced functionality has not been addressed in prior art implementations.
In the course of specifying a preferred implementation of the aforementioned functionality, an approach to receiving differential phase-shift keyed signals of arbitrary density is described. Such a receiver is useful specifically for the type of optical signals generated by the AlphaEta encryption protocol, but is generally applicable for any optical communication system based on advanced modulation formats such as differential quadrature phase shift keying (DQPSK), especially in a wavelength division multiplexed (WDM) environment. In particular, a method of using just one optical interferometer to measure multiple signals without requiring frequency locking is disclosed.